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INA700EVM: Small power measurement with INA700EVM

William Eyvaz
Prodigy 10 points
Part Number: INA700EVM
Other Parts Discussed in Thread: INA700, INA232, INA238, INA228

Tool/software:

Hello,

I am trying to measure current and power of a voltage rail (~3V) with INA700EVM, the average current level is in between 1 - 6mA. Based on the simulation results given to us, there should be around 1mW or less. Is it possible to use this INA700EVM? If so what would be the best setting for the conversion delay to set up for about 4K-5K samples. I appreciate if you can give us some advise. I also have a INA232EVM and installed a 12mΩ current sense resistor. based on the  

Thanks,

William

  • 0
    Prodigy 10 points

    Sorry, for the confusion on the current levels, it should be around 250uA -300uA. 

  • 0 in reply to William Eyvaz
    TI__Mastermind 30025 points

    Welcome to the forum,

    I will respond promptly.

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    TI__Mastermind 30025 points

    Hello Valued Engineer,

    Using the INA700 to measure 250 uA will be challenging. The device has a rated offset to +/-1.5mA, so you would need to perform a 1-point offset calibration at least and then configure device to measure with long conversion times and many averages just to filter out noise, which would reduce signal bandwidth significantly. This is something that would need to be validated on bench to confirm noise of your load and system could be digitally filtered out. Also note that best current LSB is 480 uA.

    Theoretically after you calibrate out initial offset and filter out noise, you are left with 0.5% gain error. However you will still have offset from system ambient temperature drift. Also, the input bias currents will generate an offset that will vary based upon bus voltage (Vcm) and temperature. See figures 6-13 and 6-14 of datasheet. 

    As for the INA232 + 12mOhm shunt, you will also need to perform a 1-point offset calibration. At 250uA, Vshunt is 3uV, but device offset is rated to +/- 50uV.

    The current LSB is better at 625nV/12mOhm = 52.1 uA, but you will still need to perform significant averaging and/or long conversion times given how small shunt voltages are. After this you would be left with the 0.3% device gain error plus the shunt resistor tolerance and thermal drift which is the disadvantage of INA232 compared to integrated solution like INA700. The last advantage to INA232 is it has low input bias current in nano Amps so this will not create a variable offset over Vcm.

    If the INA700s LSB is too high, then I would recommend something with better resolution, either INA232 or even INA238 or INA228.

    I would increase your shunt resistance as large as system can allow and then program firmware to perform a 1-point offset calibration after power up (basically just record current or shunt into host memory and then subtract this from all subsequent measurements).

    The increase average or conversion times are much as possible so you still can satisfy the required signal bandwidth of system. If you need a fast response to an ALERT condition, then maximize averages and minimize conversion times because ALERT conditions are checked after each average. See Figure 7-2 of INA232 datasheet.

    Hope this all makes sense.

    Sincerely,

    Peter